One system and method for processing engine exhaust gas emissions is described in U.S. Pat. No. 6,581,370. The patent describes a single exhaust system having two different passages that are capable of treating engine emissions in different ways. This arrangement provides different ways to treat engine exhaust emissions for different engine operating conditions. In particular, the patent describes a system wherein exhaust gases are directed through a passage that contains a hydrocarbon trap when the exhaust system temperature is low. Then, when the humidity of exhaust gas downstream of a hydrocarbon trap exceeds a predetermined amount, engine exhaust gases are redirected through a passage that has no emissions control device. In this way, the system purportedly reduces engine hydrocarbon emissions.
The above-mentioned system can also have several disadvantages. Specifically, the system simply decides when to switch between exhaust passages in response to humidity sensed downstream of the hydrocarbon trap. While this may be adequate for a single type of fuel, it may be less effective for different types of fuels that produce different concentrations of water vapor after being combusted. Specifically, when alcohol based fuels such as ethanol are combusted they produce higher water concentrations in the exhaust gases. Since traps tend to be less than 100% efficient, combusting ethanol can increase the humidity level observed in the exhaust gases after the hydrocarbon trap as compared to the humidity produced when gasoline is combusted. This condition may cause the system to switch between exhaust paths before the trapping cycle is complete, thereby reducing hydrocarbon conversion efficiency. Further, the increased sensed humidity may cause the system to believe that the hydrocarbon trap has degraded since trapping time may be reduced.
The inventors herein have recognized the above-mentioned disadvantages and have developed a system and method that offers substantial improvements.